Based on your analysis that it would be the only 3 digit standard, it certainly would be possible and not too hard to parse day of year, but if it's not a standard then it would rarely be used and would needlessly complicate the time code.

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While there is a reference to yearDay in both ISO 8601 and RFC3339 as @ulikunitz stated, the original reason for requesting this feature has to do with yearDay pervasiveness in the space industry. Parsing of yearDay is lacking in the time package and I thought that would be something that might be needed/useful as this is built-in in other modern languages.

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Using 002 would change the output of more-or-less meaningful date formats like Jan 002 which currently print a three digit day-of-month. While I don't know why someone would do that it doesn't seem impossible. Using 888 in a current date string seems less likely.

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@robpike and I discussed this and agreed to try 002 early in the Go 1.12 cycle. I'll send a CL shortly. After adding this to Format and Parse the obvious hole in the API is some way to take a year+yday and turn it into a year/month/day, short of preparing a string and calling Parse.